1. Field of the Invention
This invention relates to free radical polymerization. More particularly it relates to the free radical polymerization of vinyl monomers, especially copolymers of vinyl chloride. Furthermore it relates to the control of free radical copolymerization in emulsion of vinyl chloride, when a bis(hydrocarbyl)vinylphosphonate is a comonomer.
Emulsion polymerization of vinyl monomers generates large amounts of heat usually 18 to 25 kilocalories per mole, termed by those in the art as the "exotherm". If the exotherm is not carefully controlled, the temperature of the reaction can rapidly escalate into an explosion. At the commercial level of batchwise production, 1000 to about 250,000 liters per batch, the temperature of the contents in the reactors is controlled by applying either steam to heat or cold water to cool jacketed reactors.
Temperature control by heating or cooling means surrounding the reactor depends on good heat transfer through the walls of the reactor. Any factor inhibiting good heat transfer, therefore makes large scale vinyl polymerization potentially dangerous.
Before the instant invention, copolymerization of a bis(hydrocarbyl)vinylphosphonate with vinyl chloride and vinyl acetate to make useful vinyl resins generated a rapid rise in viscosity which caused the resin to coat out on the walls of the reactor. This coating lowers the heat transfer characteristics of the reactor to such an extent that temperature control becomes difficult. Absent the control of temperature, the copolymerization process is hazardous when scaled up to the level of 1000 liters or more, restricting production to laboratory (less than 20 liters) or pilot plant scale (20 to 80 liters). With the use of the process of this invention, commercial copolymerization of a bis(hydrocarbyl)vinylphosphonate with vinyl chloride and vinyl acetate may be carried out in batchwise reactors of up to 250,000-liter capacity.
Vinyl chloride/vinyl acetate/bis (hydrocarbyl)vinylphosphonate copolymers have a low fusion temperature so that laminates can be made with fabrics or substrates of comparatively low fusion points themselves, e.g. polypropylene. The bis(beta-chloroethyl)vinylphosphonate comonomer, "bis beta", imparts a rubbery nature to the resin. It is precisely this rubbery attribute which causes the polymerizing latex particles to stick to each other and to the walls of the vessel, reducing the heat transfer coefficients of jacketed reactors. Furthermore, the exotherm in the copolymerization of "bis-beta" with vinyl chloride occurs earlier, at a lower conversion, than does the exotherm for the homopolymerization of vinyl chloride alone. The peak of the exotherm for the homopolymerization of vinyl chloride in emulsion or suspension comes at about 70% conversion. The exotherm for the copolymerization of "bis beta" with vinyl chloride comes at about 30-35% conversion, which makes the reaction more difficult to control. The rate of polymerization of the "bis beta" alone or with vinyl chloride is faster than that of vinyl chloride alone.
2. Description of the Prior Art
In U.S. Pat. No. 4,097,663 granted to Gallagher and Goswami on June 27, 1978 a process for the copolymerization of 75 to 90% vinyl chloride, 5 to 15% vinyl acetate, and 5 to 15% bis(hydrocarbyl)vinylphosphonate is disclosed. The low fusion resins of Gallagher et al are useful in making cellular polyvinyl chloride laminates of the type disclosed in U.S. Pat. No. 3,983,294 granted to Goswami on Sept. 28, 1976.
Gulbins et al were granted U.S. Pat. No. 3,781,243 on Dec. 25, 1973 for a process of controlling the temperature in vinyl chloride, vinylidene chloride and vinyl acetate polymerization. Their controlling additive is a copolymerizable additive which has a copolymerization parameter r.sub.1 (r.sub.R) greater or equal to 1, a copolymerization parameter r.sub.2 (r.sub.M) less than 1, and a reactivity ratio of these two numbers greater than 20.
In U.S. Pat. No. 3,560,462 granted to Farber et al on Feb. 2, 1971 a method for controlling the rate of polymerization of vinyl chloride alone is given. The rate is speeded up in the first half of the polymerization and slowed down in the second half by the addition of a long-chain alpha-olefin.
As reported in Chemical Abstracts 64, 12796 (1966), Tudos and other Hungarian workers inhibit the homopolymerization of vinyl acetate by adding aromatic comonomers.